Various sintering compositions for the silicon nitride sintered bodies are well known: such as silicon nitride/oxide of rare earth element/aluminum oxide system; silicon nitride/yttrium oxide/aluminum oxide/aluminum nitride system; and silicon nitride/oxide of rare earth element/aluminum oxide/titanium oxide system or the like. Sintering assistant agents composed of the oxides of rare earth elements, such as yttrium oxide (Y2O3) in the sintering compositions listed above, have a function of generating grain boundary phase (liquid phase) composed of Si-rare earth element-Al—O—N or the like during the sintering operation. Therefore, the sintering assistant agents are added to a material composition for enhancing the sintering characteristics of sintering materials, and achieve high density and high strength of the sintered bodies.
According to the conventional art, the silicon nitride sintered bodies are generally mass-produced as follows. After a sintering assistant agent as mentioned above is added to the material powder of silicon nitride, the material mixture is molded to form a compact. Thus obtained compact is then sintered in a sintering furnace at a high temperature of about 1,700–1,900° C. for a predetermined period of time.
However, in the conventional manufacturing method described above, since the sintering temperature was greatly high to be about 1700 to 1900° C., there had been raised the following problems. That is, it was required to upgrade a heat-resistant specification for the sintering furnace and ancillary equipments thereof, so that an installation cost of the manufacturing facilities was greatly increased. Further, it was difficult to adopt a continuous manufacturing process, so that a manufacturing cost of the silicon nitride sintered body was remarkably increased and a mass-productivity of the sintered body was disadvantageously lowered.
In addition, although the silicon nitride sintered body produced by the conventional method achieves an improved bending strength, fracture toughness and wear resistance, however, the improvement is insufficient. A durability as a rolling bearing member requiring a particularly excellent sliding property is insufficient, so that a further improvement has been demanded.
In these days, a demand of ceramic material as precision device members has increased. In these applications, advantages such as high hardness and light weight together with high corrosion resistance and low thermal expansion property of the ceramic are utilized. In particular, in view of the high-hardness and high wear resistance, application as a wear resistant member for constituting a sliding portion of the bearing or the like has been rapidly extended.
However, in a case where rolling balls of a bearing or the like were constituted by the wear resistant member composed of ceramic, when the rolling balls were rolled while being repeatedly contacted with counterpart at a high stress level, the rolling life of the wear resistant member was not sufficient yet. Therefore, a surface of the wear resistant member is peeled off and the member causes cracks, so that the defective member was liable to causes vibration and damage to a device equipped with the bearing. At any rate, there had been posed a problem that the durability and reliability as a material for constituting the parts of the device was low.
The present invention had been achieved for solving the aforementioned problems. Accordingly, an object of the present invention is to provide a wear resistant member and a method of manufacturing the member excellent in wear resistance, particularly excellent in rolling life characteristics that are suitable as rolling bearing member, in addition to a high density equal to or higher than the conventional silicon nitride sintered body and a high mechanical strength which is inherent to the silicon nitride sintered body, even if the sintered body is manufactured through a sintering operation under a low temperature of 1600° C. or lower.